1. Field of the Invention
The present invention relates generally to a tanker. More particularly, the present invention relates to a cargo oil spillage preventive type tanker which assures that cargo oil spillage can reliably be prevented in the event of damage or injury on a part of the ship body due to stranding or similar malfunction. Further, the present invention relates to a cargo oil spillage preventive type tanker including degassing means for discharging an inert gas filled in each cargo oil tank to the outside after cargo oil is loaded.
2. Description of Related Art
A conventional tanker has been heretofore equipped with a predetermined number of separate ballast tanks (each of which is not practically used as a cargo oil tank) in a cargo oil tank section in the ship body in conformity with the regulations relating to the prevention of oil pollution. Specifically, to minimize outflow of a cargo oil in the event of stranding, collision or similar malfunction, a certain section such as separate ballast tanks exclusive of cargo oil tanks is arranged within the range defined by an outer plate of the ship's side wall and an outer plate of the ship bottom so as to protect the cargo oil tanks from damage or injury.
To facilitate understanding of the present invention, typical conventional tankers will briefly be described below with reference to FIGS. 39 to 45 each of which schematically illustrates arrangement of cargo oil tanks.
FIGS. 39, 40 and 41 schematically show a conventional tanker which includes a plurality of ballast tanks 10 and a plurality of cargo oil tanks 12 alternately arranged along the ship's side walls, respectively. With such construction, however, a cargo oil flows out from the cargo oil tank 12 arranged along the ship's side wall when the cargo oil tank 12 is damaged or injured due to a collision or similar malfunction. In addition, a cargo oil flows out from the cargo oil tank 12 when the bottom wall of the tank 12 is damaged or injured.
Next, FIG. 42 and FIG. 43 schematically show a conventional tanker which includes a plurality of double-sided hull constructions. Since a plurality of ballast tanks 4 are arranged along the full length of the ship's side walls, outflow of a cargo oil can be prevented when the ship's side wall is damaged or injured. However, if the bottom wall of a cargo oil tank 12 is damaged or injured, a cargo oil unavoidably flows out through the damaged or injured part on the bottom wall.
Next, FIG. 44 and FIG. 45 schematically show a conventional tanker which includes double bottom constructions. Since a plurality of ballast tanks 11 are arranged along the whole area of the ship bottom, outflow of a cargo oil from cargo oil tanks 12 can be prevented in the event of damage or injury on the ship bottom. However, if a part of the ship's side wall is damaged or injured, a cargo oil unavoidably flows out through the damaged or injured part on the ship's side wall.
As described above, any one of the conventional tankers cannot effectively prevent outflow of a cargo oil when a part of the ship's side wall or the ship bottom is damaged or injured for some reason. To obviate this problem, development efforts have been conducted to provide a tanker including double hull constructions and a tanker including a horizontal bulkhead, as described in the following.
FIG. 46 and FIG. 47 schematically show a tanker including double hull constructions. The interior of each double hull construction serves as a ballast tank 12. Since each cargo oil tank 12 is covered with the ballast tank 13, the entirety of the ship's side walls and the ship bottom is protected from damage or injury by the presence of a plurality of ballast tanks 13. In view of the wide area occupied by the ballast tanks 13, each double hull construction may be dimensioned to be thin in thickness and still can hold a necessary quantity of ballast. In addition, a smallest value within the specified range may be employed for the thickness of each double hull construction from the viewpoint of building a tanker at low cost and cruising it economically. Pursuant to the current provisions in Japanese oil pollution prevention law, the thickness of each ballast tank 13 has to be larger than a smaller value of either 1/15 of a width of the ship body or 2 meters. Therefore, in practice, the thickness of the ballast tank 13 is selectively determined in consideration of the aforementioned conditions.
When a tanker is built with double hull constructions, the thickness of each double hull construction is usually dimensioned to be less than the thickness of a double-sided wall structure in the conventional tanker which is built with double-sided hull constructions. Therefore, although not only the side walls of the ship but also the ship bottom are protected from damage or injury in the presence of a plurality of ballast tanks 13, there is a high possibility that the inner hull of each double hull construction is damaged or injured when an outer plate of the ship's side wall or the ship bottom is damaged or injured by a large shock, because each ballast tank 13 is dimensioned to have a thin thickness. In such case, outflow of a cargo oil from the damaged or injured part of the cargo oil tank 12 is unavoidable.
Next, FIG. 48 and FIG. 49 schematically show a conventional tanker which includes a mid-height deck 3 as a horizontal bulkhead in a cargo oil tank section. FIG. 48 is a cross-sectional view of the conventional tanker similar to a tanker shown in FIG. 4 to FIG. 6, particularly illustrating by way of example arrangement of the mid-height deck 3 as a horizontal bulkhead in the cargo oil tank section. Each of cargo oil tanks 16 to 18 and 24 is filled with a cargo oil 14.
FIG. 49 is a cross-sectional view of the conventional tanker, schematically illustrating by way of example a structure in which a plurality of cargo oil tanks arranged along the ship's side walls in the region above the mid-height deck 3 serving as a horizontal bulkhead are practically used as ballast tanks 15 and all the remaining cargo oil tanks 16 to 18 are used as actual cargo oil tanks.
In a case where a conventional tanker includes a mid-height deck in the above-described manner, the mid-height deck 3 has been inadequately arranged such that it is located at a position lower than a highest draft line 19 in such a manner as to divide the interior of the ship's body into two halves as seen on a transversal plane (see FIG. 48) or it is located at a position directly below the highest draft line 19 (see FIG. 49).
However, when a tanker cruises in practical operation, there arises a case where the tanker cruises with only some of cargo oil tanks filled by a cargo oil. In this case, a draft line is lowered from the highest draft line 19, and the tanker cruises with a draft corresponding to about a half of the depth of the ship's body. When the draft line during practical cruising of the tanker is lowered below the position of the mid-height deck 3, the mid-height deck 3 does not contribute to the prevention of undesirable outflow of a cargo oil in the event of damage or injury to the ship bottom, as described later.
In a case of the conventional tanker including a mid-height deck, e.g., the tanker shown in FIG. 48, the outflow of a cargo oil in the event of damage or injury to the ship bottom can be prevented. However, when the ship's side wall is damaged or injured, the outflow of the cargo oil 14 cannot be prevented.
The tanker shown in FIG. 49 has an advantage that outflow of a cargo oil can be prevented in the same manner as the tanker shown in FIG. 48 when the ship bottom is damaged or injured. However, when the ship's side wall is damaged or injured, especially when a lower cargo oil tank arranged along the ship's side wall is damaged or injured, outflow of the cargo oil 14 cannot be prevented. In FIG. 49, reference numeral 5 designates an air venting tube.
FIG. 50 is a cross-sectional view of the conventional tanker in FIG. 49, particularly illustrating a process of outflow of the cargo oil 14 when a lower cargo oil tank 16 arranged along the ship's side wall is damaged or injured.
In the initial state A immediately after an occurrence of the damage or injury as shown in FIG. 50, since the pressure of sea water is higher than the pressure of a cargo oil 14 in the damaged or injured lower cargo oil tank 16, sea water flows in the lower cargo oil tank 16. This causes the cargo oil 14 in the lower cargo oil tank 16 to be forcibly displaced in the upward direction through the air venting tube 5.
Next, in the intermediate state B at time T.sub.1 after occurrence of the damage or injury, the pressure of the sea water which has flowed in the bottom part of the lower cargo oil tank 16 becomes equalized to the pressure of the cargo oil 14 which has been forcibly displaced in the upward direction through the air venting tube 5. At this time, the replacement of the cargo oil 14 having a lower specific weight with the sea water having a higher specific weight begins. In the final state C after time T.sub.2, the surface level of the sea water which has flowed in the lower cargo oil tank 16 reaches the upper end of the damage or injured part of the lower cargo oil tank 16. At this time, the outflow of the cargo oil 14 stops and this balanced state is maintained between the cargo oil 14 and the sea water.
In the case shown in FIG. 50, the draft line 19 before an occurrence of damage or injury is located upward of the position of the mid-height deck 3. Thus, a part of the cargo oil 14 in the lower cargo oil tank 16 does not flow out to the surface of the sea but remains in the lower cargo oil tank 16. However, in a case where the draft line is located lower than the position of the mid-height deck 3, the entire cargo oil 14 in the damaged or injured lower cargo oil tank 16 flows out.
As is apparent from the above description, every one of the conventional tankers has the problem that it can not sufficiently prevent the outflow of a cargo oil in the event of damage or injury on a part of the ship body.
In addition, in a case where the mid-height deck 3 serving as a horizontal bulkhead is arranged in the cargo oil tank section as shown in FIG. 48 and FIG. 49, if a degassing means exclusively employable for replacing an inert gas in each lower cargo oil tank with fresh air after the completion of a cargo oil loading operation is disposed for each of the lower cargo oil tanks, this leads to another problem that the tanker has to be built at a substantially increased cost, because the tanker unavoidably includes many lower cargo oil tanks.